Make a subject access request under the data protection act to CAFCASS. Information sheet here.

You might find it useful to do a subject access request to the school as well - the ICO has a factsheet here. Other public bodies might hold useful information too, e.g. GP surgery or clinic for health notes.

They may or may not have kept records that far back but these requests will allow you access to the whole file and you can see what information was kept.

Social workers don't actually ever get rid of the records. They may be filed in a dark basement somewhere, and they may be filed in the wrong place and lost, but our city council has files dating back to the dark ages.